IJETAE_1112_64

7/27/2019 IJETAE_1112_64

http://slidepdf.com/reader/full/ijetae111264 1/7

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 11, November 2012)

408

Calculation of the Factor of Stress Concentration in theJunction’s Welded Tube Subjected to Combined Loadings

A. Fouathia1, A. Mekroud

2, A. Benmeddour

3, K. Bellagh

4

1,2,3,4 Mechanics Laboratory, Faculty of Engineering, University Mentouri-constantine Chaab Ersas Campus, 25000

Constantine Algeria

Abstract — Fatigue failure caused by stress

concentrations in welded tubular joints is observed in the

offshore platforms subjected to cyclic loading in corrosive

marine environments. Therefore, it is necessary to

accurately assess the intensity of stress concentration to

properly address the problem of fatigue damage, and result

in reliable tubular junctions. Stress concentrations in theoffshore structures usually occur at the intersections of

tubular joints. In some junctions, the stress concentration

may induce a maximum stress at the intersection 30 times

the nominal stress, and increase the risk of fatigue failure in

tubular joints. This work aims to study the stress

distribution points and "hot" in the tubular welded joints, subjected to combined loading of traction and rotating

bending (bending in the plane, out of plane bending and

traction)simulating better loading real.

Keywords — stress concentration, offshore structures,

finite element method; tubular welded joints.

I. I NTRODUCTION

Welded tubular assemblies are widely used in steel

construction in the modern bridges, towers and offshore

structures such as oil rigs Offshore Jacket style for the

exploitation of hydrocarbon reserves in the marine

environment.They are classified according to their form

in T. Y. X, K, DT, DY, and DK [1].

The welded the frames with the amounts and diagonalforms a node characterized by a variable stiffness, non-

uniform distribution of stresses and a complex three-

dimensional behavior, while this may be unfavorable if

the assembly is poorly designed or poorly made and can

lead to fatigue failure resulting from cyclic loading to

which the structure is subjected[2]. Offshore platforms

are subjected during their service life (20-30 years) to

various environ mental actions; waves, currents, wind

and storms are particularly severe suffering and the

combined action of several stresses including traction

[3]. Bending in the plane and bending out of plane, the

stresses at the junctions of tubes create hot spots or areas

of high stress concentrations [4]. In some junctions, the

stress concentration may induce a maximum stress at the

intersection 30 times the nominal stress inevitably

leading to a fatigue damage of these structures [5].

Besides the security problem they pose, the fatigue

failure cause significant operating losses for the user andcostly to repair and redesign high for the manufacturer

[6].

It is therefore necessary to assess accurately the

intensity of stress concentration to properly address the

problem of fatigue damage, and result in reliable tubular

junction, and it will pay particular attention to the design

and the achievement of welded assemblies.

So far, research [7] in the field of fatigue weldedtubular nodes was conducted mainly conducted by the

offshore oil industry[8].

This study discusses the phenomena of fatigue in the

nodes of T tube assemblies welded metal structures

subject to marine stress due to random natural elements

(waves, wind, current, ...)

Its purpose is to study the stress distribution and

location of points "hot" (hot-spot stresses) at critical points in terms of fatigue in welded tubular joints

subjected to static loading along the traction, bending in

the plane, bending out of plane and the combination:

tension / bending in the plane and traction / bending out

of plane loading simulating more real it is to find anumerical simulation tool to study and predict the

behavior of welded tubular structures, a simple and

accurate modeling of the structure using a computer code

based on the finite element method that will calculate the

stress concentration near the weld. A mixed formulation

will ensure a balance of efforts at the junctions locally

and globally, on the whole structure.

II. R ESULTS A ND DISCUSSION

1 Stress distribution

The constraints considered in this study will be

calculated at the centroid of each finite element, so as tohave an average value of the stress tensor finite element

considered. These are the Von Mises stresses.

The results of the simulation of the load of the tubular

structure welded in tension we can take the existence of

two zones in the spacer:

• area 1 where the stresses decrease abruptly, is located

above the weld and in the immediate vicinity of the latter,

• area 2 in which the stresses gradually decrease, while

having a small change from place to reach values nearly

constant at the upper end of the brace, figure 1.

σN2 is the nominal stress measured in zone 2.

σN1, σN2: nominal stresses sampled at the edge of the

“hot” area located on the extension, above the two hotspots related to two points of crown point at a height ofabout (D + d) / 2 from the point of saddle.

7/27/2019 IJETAE_1112_64




409

Σmax is chosen as the highest values of σN1, σN2.

Either: σmax = sup (σN1, σN2) (1)

Figure1: Areas of stress distribution

In figure 2, which shows the evolution of the Von

Mises stress along the periphery of the brace, there is the

presence of the two areas already explained:

Figure 2: Distribution of stress on the brace.

By applying a traction force on the brace, the structure

undergoes a deformation whose general form is shown inFigure3, we see that the deformation is largely located in

the middle of the chord just below the brace; the latter

also undergoes a strain but lower proportion to the chord.

The deformation of the latter is mainly longitudinal.

Figure 3: Field of stresses in the structure with distorted.

2.1 Constraints in the junction

In the case of traction stress concentration around the

junction is divided symmetrically into two points and

this, it is largely visible in the vicinity of two points of

the crown favors the appearance of the hot spot in the

two points. In Figure 4 we see this stress concentration.

The maximum point is 32.4 Mpa High Chairs and the

minimum value is 0.11 Mpa.

Figure 4: Concentration of stresses in the Weld.

For reasons of symmetry, we consider only half of our

welded structure, so the study will focus on the portion

between the saddle point (Φ=0°) and crown point

(Φ=90°).

The figures, 5a and 5b show the distribution of stress

intensity factor kt in the structure. It is clear that the

concentration increases gradually from the point of

saddle until it reaches the maximum value at the crown

point (kt = 6.4). In Figure 5b best seen changes in the

stress concentration factor based on the positioning angle

Φi between two points saddle and crown, this explains

the increase in concentration.

7/27/2019 IJETAE_1112_64




410

Figure 5 a: stress intensity factor in traction

Figure 5b: Change kt according Φi

2.2 Out of plane Bending

In this case, the structure will be stressed in bending

out of plane this solicitation will result in a transverse

deformation which will be located primarily on the

frame, with a party that will undergo a pull and the othera compression figure 6.

Figure 6: stress concentration in bending out of plane

2.2.1. Constraints at the junction

In this case load, we can distinguish a similarity in the

distribution of stress concentration with the traction that

is to say that the concentration is on two points of crown,

see figure 7. The concentration changes in the same way

as in the case of traction, except that the values are lower

compared to those of traction, again for reasons ofsymmetry we consider only the portion between the

saddle point and crownpoint.

The minimum value is always at the saddle point at the

angle Φ=0°, while the maximum is at a crown point

(Φ=90°), and is in the range of 296.7Mpa.

Note that in the case of OPB, the value of the stress of

the hot spot is larger than that of the traction.

Figure7: Distribution of stress in the welded joint.

The distribution of stress intensity factor kt throughout

the tubular structure is representing in figure 8a. The

curve in figure 8b representing the evolution of the stress

intensity factor kt based on the angle of position Φi, takes

the same shape as the curve of traction. Concentration

increases gradually from the saddle point until it reaches

the maximum value at the crown point about 7.8 Mpa.

Figure 8a: stress intensity factor in traction

7/27/2019 IJETAE_1112_64




411

Figure 8b: Curve of variation of kt based Φi

2.3. Bending in the plane

We apply in this case a bending in the plane of the

frame creating a deformation made by compression on

one side and pulling the other. Note that most of thedeformation is located on the chord near the junction

between the two tubes, the spacer undergoes

deformation, but also to a lesser extent, figure 9.

Figure 9: Distribution of stresses in the structure

2.3.1. Constraints at the junction

From Figure 10, which shows the distribution of

stresses in the structure (the case of IPD) unlike the two

previous cases of loading, the stress concentration is notlocated very near the crown but is diverted to the saddle

point. It is in the range of 141.6Mpa.

In both previous cases we distinguish the presence of

two hot spots; in the case of the IPD we also notice the

same thing.

The symmetry in the distribution of the concentration

still exists.

Figure 10: Concentration of bending stresses in the plane.

The concentration in this case, changing gradually

from the saddle point to a maximum value (kt = 5.06) at

an angle Φ=90° and then begins to decrease until crown

point. We note here that the hot spot is not located at the

crown point as the other two cases, but in the saddle

point figure 11.

Figure 11: Factor intensity of traction stresses.

In the same way as in the other two loads we draw the

curve representing the evolution of the stress intensity

factor based on, but this time the shape of the curve is

completely different from the other two. But we can see

the appearance of symmetry of this curve.

7/27/2019 IJETAE_1112_64




412

Figure 12: Curve of variation of Kt based Φi Solicitation of the

structure IPD

2.4. Comparison of results

In Table 1 we compared the results of our calculationsand those obtained by parametric formulas and numerical

calculations of different researchers [9], a structure that

represents the same dimensions.

Load

cases

Kuang

& al.Gibstein

Hel.

& alF.E.M

our

calculat-ion

TRAC

-TION 9.6 11.2 6.7 8.6 6.48

IPD 3.3 3.4 2.8 3.2 5.06

OPD 8.6 11.3 6.1 9.1 7.81

Table 1: Comparison of results

We find that the results of our calculations are closer

to those obtained in particular by Hel. and al, in general,

by other researchers, for the same loads.

III. COMBINED LOADING

3.1. Combination pull-OPD-IPD

Applying a combined loading of traction and bending

out of plane and bending in plane with the same load of 4

Mpa. Note that the stress concentration at the junction

between the chord and the brace is located in the vicinity

in two points of crown. Given the combination of loads,

the chord is extremely distorted.

Figure 13: Concentration of stresses in traction-IPD-OPD.

Figure 14: distribution of stress in the welded joint.

This stress concentration is not symmetric. It startswith a maximum value 6.3 to crown point, Φ=0° angle

and begins to increase until it reaches the value of 10.3 at

Φ=72° angle, then the constraints will gradually decrease

until it reaches the value of kt = 3.6 to angle Φ = 143 °,

from that point until it increases the value of 6.3 kt in the

second crown point at the angle Φ=180°.The graph representing the evolution of the stress

concentration factor kt based on the positioning angle is

shown in Figure 15.

Figure 15: Factor intensity of traction

7/27/2019 IJETAE_1112_64




413

Figure 16: Curve of variation of Kt based Φi.

IV. DISCUSSED A ND R ECOMMENDATIONS

4.1. In case traction

Constraints prevailing at the junction chord-brace in a

tubular T as an axial stress produced by bending, due to

the overall deformation of the chord. Any changes to

these constraints will have a significant influence on the

values of the Kt and reducing the deformation of thechord.

The best way to reduce the deformation of the chord is

to add a stiffener where the chord is deformed. Research

[10] [11] has proven that this method can reduce themaximum hot spot up to 40%. Following his research,

Baker gardening came to reduce the maximum hot spotof 5.25 to 3.15, using this principle.

4.1.1. In case of bending in the plane

In the case of bending in the plane, the stresses at the

junction spacer ribs are produced by local bending of the

chord.

By acting the same way as in the case of traction, that is

to say by adding a stiffener in the chord, you can get a

slight reduction in the maximum value of kt, however,

this value is low given the meaning deformation of the

chord.

4.1.2. In case of bending out of planeBending out of plane is similar to that of traction when

the overall deformation is the main parameter generating

constraints at the junction spacer chord.

As in the case of traction, any reduction in the overall

deformation results in reduced values of the Kt.

With the same principle of traction as stiffening, themaximum values of kt can be reduced to 30%.

The calculation of stress concentration factors can

provide some information on the distribution of stresses

in the structure studied in the type of solicitations.

Given the complexity due to many parameters and in

particular the geometry of the intersection curve, there is

no straightforward method to take into account the stressdistribution near the weld, only the method of FEA can

make any part of the answer.

Some research work, namely those conducted by the

CTICM [12] (Industrial Technical Centre of MetalConstruction, France), have established the variation of

stress concentration factor according to each geometric

parameter, as a function exponential type.

The work carried out on cylindrical notched loaded in

tension, have to take into account the exponentialincrease of the stress distribution.

The fact remains that this possibility is being ruled out

of tubular structures, except in tension [13].

Therefore, the expression of stress concentration factor is

given by the ratio of the maximum stress σmax to the

hottest point on the stress nominal σnom near the weld, both on the side of the spacer as that of the chord. We

consider for each finite element of the structure, and for

each stress, the Von Mises stresses, in order to obtain the

stress concentration factors respectively. Constraints are

taken into account the constraints of so-called "skin" for

thin shell elements [12].But we can say to the existence of a zone of symmetry

between the saddle point and the crown point; in this area

the factors of stress concentration are identical, and are

found to increase with as we advance the saddle point to

crown point. The hot spot changes position depending on

the geometry of the node and the nature of the

solicitation.

V. CONCLUSION

Use of COMSOL Multiphysics code calculation based

on the finite element method to study and predict the

behavior of the tubular structure welded T-shaped a

simple and accurate modeling of this software. To

calculate the stress concentration (the factors of stressconcentration) in the vicinity of the weld and also to

locate hot spots or areas of high stress concentrations.

Given the complexity and nodes tubular geometric

discontinuity, this method seems best suited,

This study has shown that hot spots are generally located

in the crown points: traction/bending out of the plane and

combined traction / bending out of plane with values ofthe factors of stress concentration which can reach

respectively: ( 8.99, 10.36, 13.98) and are satisfactory

compared to those found by other researchers (9.62,

10.45, 18.0) [13][14], except in the vicinity: in plane bending and the combination: traction/bending in the

plane with values respectively as : (4.94 to 58.5°, 11.01

to 72°) and that approach also values obtained by the

searches: (3.11 at 45°, 9.8 to 67.5°), based on these

values shows that the maximum stresses in the hotspots

also vary depending on the load. The computer code can be used to achieve consistent results and consistent with

the literature.

7/27/2019 IJETAE_1112_64




414

REFERENCES

[1 ] F.Ghanameh, D.Thevenet, A.Zeghloul "Concentration de

contraintes dans les jonctions tubulaire de type T, Y et K" 17eme

congré de mecanique-Troys septembre 2005-

[2 ] Fathi GHANAMEH : "Fiabilite des jonctions tubulaires soudees

des plateformes offshore" (2002-2005)

[3 ] S. A. KARAMANOS, A. ROMEIJN, J. WARDENER, "Stress

Concentrations in Tubular Gap K-Joint: Mechanics and Fatigue

Design", Engng, struct. vol.22 , 4-14, 2000.

[4 ] VISSER W, "On the structural design of tubular joints", Offshore

Technology Conference, OTC 2117, Houston, Texas (1974).

[5 ] American Welding Society, "Structural Welding Code-Steel",

14th edn, ANSI/AWS, Miami (1994).

[6 ] American Petroleum Institute, "Recommended Practice for planning, Designing and constructing Fixed Offshore Platforms",

1st edn, API RP2a-LRFD, Washignton DC (1993).[7 ] Lloyd's register of shipping. "Stress Concentration Factors for

Simple Tubular Joints", HSE BOOKS, OTH 354 (1997).

[8 ] Shao Yong-Bo "Geometrical effect on the stress distribution alongweld toe for tubular T- and K-joints under axial loading" Science

direct Journal of Constructional Steel Research 63 (2007)

1351 – 1360

[9 ] A. N’Diaye, S. Hariri, G. Pluvinage and Z. Azari . “Stress

concentration factor analysis for welded, notched tubular T-jointsunder combined axial, bending and dynamic loading”

International Journal of Fatigue, Volume 31, Issue 2, February2009, Pages 367-374

[10 ] M.M.K. Lee, D. Bowness. “Estimation of stress intensity factorsolutions for weld toe cracks in offshore tubular joint ”.

International Journal of Fatigue 24 (2002) 861 – 875

[11 ] CEE 504: Finite Element Methods in Structural Mechanics Winter2009 Using COMSOL Multiphysics to solve structural mechanics

problems.

[12 ] Rapport CNEXO-CTICM, "Calcul Statique des Assemblages

Tubulaires, Concentration des Contraintes dans les Assemblages

Tubulaires".

[13 ] E. CHANG and W.D. DOVER, "Stress Concentration Factor

Parametric Equations for Tubular X and DT Joints", Int. J.

Fatigue, vol. 18,363-387, 1996[14 ] A.N’DIAYE, "Concentration des contraintes dans des structures

tubulaires" Thèse de doctorat (Mars 2001) Universite de Metz.

Documents

IJETAE_1112_64